1. Technical Field of the Invention
The present invention relates to landscape and agricultural irrigation and, in particular, to a system and method for controlling irrigation.
2. Description of Related Art
Automatic sprinkling systems are well known in the art. These systems typically operate in a manner where the user specifies a certain time for starting a watering cycle as well as a length of time that each specified watering cycle (in perhaps each of a number of watering zones) should last. In most situations, the user makes an initial best guess as to the watering needs of the vegetation, and sets a watering time based on that guess. Thereafter, the user monitors the health of the plants and condition of the soil in each watering zone to make adjustments to the initial guess as to watering needs and causing corresponding adjustments to the specified watering time for each zone. Ideally, an automatic sprinkler system should return to the vegetation only as much water as has been lost through either evaporation from the soil or transpiration from the plant. This manual guessing process for specifying sprinkling durations is notoriously inaccurate. In fact, some estimates indicate that automatic sprinkler users over-water their vegetation by as much as a thirty to forty percent (30-40%) factor.
The term "evapotranspiration" (ET) refers to the amount of water a plant uses or needs in order to maintain growth. The climatic information commonly used to calculate an evapotranspiration value include temperature, solar radiation, wind speed, and vapor pressure or humidity. This climatic information is generally collected by a full service weather station and processed in one of a number of known complex formulas or equations to calculate the evapotranspiration value. One example of such an equation recognized in the agriculture industry for accuracy in measuring evapotranspiration using weather station collected climatic information is the well-known Penman-Monteith or modified Penman equations (hereafter referred to as "modified Penmans").
Weather station collected climatic information that is useful for Penman-Monteith, modified Penmans or other weather station climatic information driven evapotranspiration equation processing is widely available from a number of sources including State agriculture agencies, County extension services and the Internet. However, it is recognized that since collected weather station climatic information is valid for only the precise site (or immediately surrounding geographic area) of the weather station, any Penman-Monteith, modified Penmans or other equation evapotranspiration value calculated from that weather station climatic information is similarly valid for only that specific site (and perhaps its immediately surrounding geographic area). The collected climatic information along with any evapotranspiration equation value calculated therefrom is accordingly of little (if any) accurate use to a farmer or land owner located tens or hundreds of miles away from, or perhaps nearby but at a different elevation than, the site of the weather station, or perhaps nearby but affected by localizes conditions such as man-made structures, forests and the like which affect evapotranspiration.
To make complete and accurate use of the Penman-Monteith, modified Penmans or other similar evapotranspiration equation, a person would thus have to purchase and install at their site a full service weather station capable of measuring the proper climatic information. For a small farmer or land owner, purchasing a weather station may not present a viable, economically feasible solution. It is thus further recognized that the Penman-Monteith, modified Penmans or similar equation is of little practical use to a person who cannot gain access to all the of the requisite, site specific climatic information needed for input to the equation. As such, these weather station climatic information driven equations are of great academic interest but provide little real-world benefit.
Considerable effort has accordingly been expended in developing evapotranspiration formulas which mimic the results provided by the weather station climatic information driven (e.g., Penman-Monteith, modified Penmans) equations, but do not require access to such large amounts of specific weather station collected climatic information. More specifically, there is a need for an accurate evapotranspiration formula which requires for its input data climatic information that is easily and inexpensively collectable at the specific site where the vegetation at issue is located. One such formulation comprises the Hargreaves equation as set forth below: EQU ET.sub.o =0.00009.times.RA.times.(T.degree. C.+17.8).times.TD.sup.0.50
wherein:
ET.sub.o =reference evapotranspiration (in inches of water per day); and PA1 RA=extraterrestrial radiation expressed in equivalent evaporation (in inches of water per day); PA1 ET.sub.o =reference evapotranspiration (in millimeters of water per day); and PA1 RA=extraterrestrial radiation expressed in equivalent evaporation (in millimeters of water per day);
or: EQU ET.sub.o =0.0023.times.RA.times.(T.degree.C.+17.8).times.TD.sup.0.50
wherein:
and further wherein: ##EQU1##
See, George H. Hargreaves, "Defining and Using Reference Evapotranspiration", Journal of Irrigation and Drainage Engineering, vol. 120, no. 6, November/December 1994. A primary advantage obtained from use of the Hargreaves equation is that the only climatic information that needs to be collected by the user and thereafter processed to determine a reference evapotranspiration value comprises local temperature data (high, low and differential). This data may be easily and inexpensively collected using either an automatic or a manual temperature sensing mechanism. The extraterrestrial radiation (RA) value reflects the evaporation resulting from the amount of the sun's radiation which reaches the earth's surface. While this extraterrestrial radiation caused evaporation value changes daily (due to day-to-day weather changes, the tilt in the earth's axis and the yearly rotation of the earth about the sun), it is recognized that a value calculated based on the latitude of the site provides a suitable approximation. Tables providing monthly latitude based extraterrestrial radiation value information are published and widely available from a number of sources. The remaining components of the Hargreaves equation are advantageously constants.
While the Hargreaves equation has been shown to mimic the results provided by the Penman-Monteith, modified Penmans and other recognized weather station climatic information driven evapotranspiration equations (see, George H. Hargreaves, "Defining and Using Reference Evapotranspiration", Journal of Irrigation and Drainage Engineering, vol. 120, no. 6, November/December 1994), at many geographic locations and at certain times of the year, the reference evapotranspiration values output from the Hargreaves equation are noticeably inaccurate. As such, the Hargreaves equation has not been relied upon as a practical tool for calculating evapotranspiration and specifying irrigation needs. The benefits that accrue from having to only measure temperature at the site in order to calculation evapotranspiration, however, weigh in favor of its use (subject, of course, to an improvement in its accuracy). What is needed is an apparatus and method for implementing the Hargreaves equations for more accurate calculation of evapotranspiration values in connection with the use of an automatic irrigation controller.